In general, it may be said that none of the glycerids capable of absorbing halogens is able to take on a quantity equivalent to theory.[293] While the saturated fatty acids (stearic series) theoretically are not able to absorb iodin some of them are found to do so to a small degree. It is evident, therefore, that it is not possible to calculate the percentage of unsaturated glycerids in a fat from their iodin number alone. According to the data worked out by Schweitzer and Lungwitz both addition and substitution of iodin take place during the reaction.[294] This fact they determined by titration with potassium iodate and iodid according to the formula 5HI + HIO₃ = 6I + 6H₂O. The authors confess that whenever free hydriodic acid is found in the mixture that iodin substitution has taken place and that for each atom of hydrogen eliminated from the fat molecule two atoms of iodin disappear, one as the substitute and the other in the form of hydriodic acid. When carbon bisulfid or tetrachlorid is used as a solvent no substitution takes place and pure additive compounds are formed.
The following process is recommended to secure a pure iodin addition to a glycerid: About one gram or a little less of the oil or fat is placed in a glass stopper flask, to which are added about seven-tenths of a gram of powdered mercuric chlorid and twenty-five cubic centimeters of a solution of iodin in carbon bisulfid. The stopper is made tight by smearing it with powdered potassium iodid, tied down, and the mixture is heated for some time under pressure. By this method it is found that no hydriodic acid is formed, and hence all the iodin which disappears is added to the molecule of the glycerid. The additive numbers obtained for some oils are appended:
| Oil. | Time of heating. | Temperature. | Per cent iodin added. | Per cent hübl number. |
| Lard oil | 30 minutes. | 50°.0 | 73.0 | 78.4 |
| Cottonseed oil | 2 hours. | 50°.0 | 103.0 | 106.5 |
| Oleic acid | 2 ” | 65°.5 | 93.8 |
331. Solution in Carbon Tetrachlorid.—Gantter has called attention to the fact that the amount of iodin absorbed by fat does not depend alone upon the proportion of iodin present but also upon the amount of mercuric chlorid in the solution.[295] Increasing amounts of mercuric chlorid cause uniformly a much greater absorption of the iodin. For this reason he proposes to discard the use of mercuric chlorid altogether for the hübl test and to use a solvent which will at the same time dissolve both the iodin and the fat. For this purpose he uses carbon tetrachlorid. The solutions are prepared as follows:
Iodin Solution.—Ten grams of iodin are dissolved in one liter of carbon tetrachlorid.
In the preparation of this solution the iodin must not be thrown directly into the flask before the addition of the tetrachlorid. Iodin dissolves very slowly in carbon tetrachlorid and the solution is made by placing it in a sufficiently large weighing glass and adding a portion of the carbon tetrachlorid thereto. The solution is facilitated by stirring with a glass rod until the added tetrachlorid is apparently charged with the dissolved iodin. The dissolved portion is then poured into a liter flask, new portions added to the iodin and this process continued until the iodin is completely dissolved, and then sufficient additional quantities of the tetrachlorid are added to fill the flask up to the mark.
332. Sodium Thiosulfate Solution.—Dissolve 19.528 grams of pure sodium thiosulfate in 1000 cubic centimeters of water. For determining the strength of the solution by titration, the solution of iodin in carbon tetrachlorid and a solution of sodium thiosulfate in water are each placed in a burette. A given volume of the iodin solution is first run into a flask with a glass stopper and afterward the sodium thiosulfate added little by little until, after a vigorous shaking, the liquid has only a little color. Some solution of starch is then added and shaken until the mixture becomes deep blue. The sodium thiosulfate solution is added drop by drop, with vigorous shaking after each addition, until the solution is completely decolorized. If both solutions have been correctly made with pure materials they will be of equal strength; that is, ten cubic centimeters of the iodin solution will be exactly decolorized by ten cubic centimeters of the sodium thiosulfate solution.
333. Method of Conducting the Absorption.—The quantity of the fat or oil employed should range from 100 to 200 milligrams, according to the absorption equivalent. These quantities should be placed in flasks with glass stoppers in the ordinary way. In the flasks are placed exactly fifty cubic centimeters of the iodin solution equivalent to 500 milligrams of iodin, and the flask is then stoppered and shaken until the fat or oil is completely dissolved. In order to avoid the volatilization of the iodin finally, sufficient water is poured into the flask to form a layer about one millimeter in thickness over the solution containing the iodin and fat. The stopper should be carefully inserted and the flask allowed to stand at rest for fifty hours.
334. Estimation of the Iodin Number.—This is determined in the usual way by titration of the amount of iodin left in excess after the absorption as above described. The iodin number is to be expressed by the number of milligrams of iodin which are absorbed by each 100 milligrams of fat.
Example.—One hundred and one milligrams of flaxseed oil were dissolved in fifty cubic centimeters of the carbon tetrachlorid solution of iodin and allowed to stand as above described for fifty hours. At the end of this time, 42.3 cubic centimeters of the sodium thiosulfate solution were required to decolorize the excess of iodin remaining.